daemon: added "group" configuration option

The "group" configuration option is similar to "user" as it
sets user set what group MPD shall run as.  With "user"
option, MPD changed GID to the GID of the user, however,
more control could be desired.

Moreover, the patch changes the way of checking whether no
setuid(2)/setgid(2) is required -- previously user names
were compered, now UID and GIDs are compered (ie. the one we
already have (getuid(2)/getgid(2)) with the one we want to
change to).

doc/mpdconf.example

@@ -61,6 +61,13 @@
 #
 #user				"nobody"
 #
+# This setting specifies the group that MPD will run as. If not specified
+# primary group of user specified with "user" setting will be used (if set).
+# This is useful if MPD needs to be a member of group such as "audio" to
+# have permission to use sound card.
+#
+#group				"nogroup"
+#
 # This setting sets the address for the daemon to listen on. Careful attention
 # should be paid if this is assigned to anything other then the default, any.
 # This setting can deny access to control of the daemon.

src/conf.c

@@ -61,6 +61,7 @@ static struct config_entry config_entries[] = {
 	{ .name = CONF_PID_FILE, false, false },
 	{ .name = CONF_STATE_FILE, false, false },
 	{ .name = CONF_USER, false, false },
+	{ .name = CONF_GROUP, false, false },
 	{ .name = CONF_BIND_TO_ADDRESS, true, false },
 	{ .name = CONF_PORT, false, false },
 	{ .name = CONF_LOG_LEVEL, false, false },

src/conf.h

@@ -34,6 +34,7 @@
 #define CONF_PID_FILE                   "pid_file"
 #define CONF_STATE_FILE                 "state_file"
 #define CONF_USER                       "user"
+#define CONF_GROUP                      "group"
 #define CONF_BIND_TO_ADDRESS            "bind_to_address"
 #define CONF_PORT                       "port"
 #define CONF_LOG_LEVEL                  "log_level"

src/daemon.c

@@ -45,14 +45,17 @@
 static char *user_name;
 
 /** the Unix user id which MPD runs as */
-static uid_t user_uid;
+static uid_t user_uid = (uid_t)-1;
 
 /** the Unix group id which MPD runs as */
-static gid_t user_gid;
+static gid_t user_gid = (pid_t)-1;
 
 /** the absolute path of the pidfile */
 static char *pidfile;
 
+/* whether "group" conf. option was given */
+static bool had_group = false;
+
 #endif
 
 void
@@ -107,16 +110,19 @@ daemonize_set_user(void)
 	if (user_name == NULL)
 		return;
 
-	/* get uid */
-	if (setgid(user_gid) == -1) {
-		g_error("cannot setgid for user \"%s\": %s",
-			user_name, g_strerror(errno));
+	/* set gid */
+	if (user_gid != (gid_t)-1 && user_gid != getgid()) {
+		if (setgid(user_gid) == -1) {
+			g_error("cannot setgid to %d: %s",
+				(int)user_gid, g_strerror(errno));
+		}
 	}
+
 #ifdef _BSD_SOURCE
 	/* init suplementary groups
 	 * (must be done before we change our uid)
 	 */
-	if (initgroups(user_name, user_gid) == -1) {
+	if (!had_group && initgroups(user_name, user_gid) == -1) {
 		g_warning("cannot init supplementary groups "
 			  "of user \"%s\": %s",
 			  user_name, g_strerror(errno));
@@ -124,7 +130,8 @@ daemonize_set_user(void)
 #endif
 
 	/* set uid */
-	if (setuid(user_uid) == -1) {
+	if (user_uid != (uid_t)-1 && user_uid != getuid() &&
+	    setuid(user_uid) == -1) {
 		g_error("cannot change to uid of user \"%s\": %s",
 			user_name, g_strerror(errno));
 	}
@@ -196,25 +203,32 @@ daemonize(bool detach)
 }
 
 void
-daemonize_init(const char *user, const char *_pidfile)
+daemonize_init(const char *user, const char *group, const char *_pidfile)
 {
 #ifndef WIN32
-	if (user != NULL && strcmp(user, g_get_user_name()) != 0) {
-		struct passwd *pwd;
-
-		user_name = g_strdup(user);
-
-		pwd = getpwnam(user_name);
-		if (pwd == NULL)
-			g_error("no such user \"%s\"", user_name);
+	if (user) {
+		struct passwd *pwd = getpwnam(user);
+		if (!pwd)
+			g_error("no such user \"%s\"", user);
 
 		user_uid = pwd->pw_uid;
 		user_gid = pwd->pw_gid;
 
+		user_name = g_strdup(user);
+
 		/* this is needed by libs such as arts */
 		g_setenv("HOME", pwd->pw_dir, true);
 	}
 
+	if (group) {
+		struct group *grp = grp = getgrnam(group);
+		if (!grp)
+			g_error("no such group \"%s\"", group);
+		user_gid = grp->gr_gid;
+		had_group = true;
+	}
+
+
 	pidfile = g_strdup(_pidfile);
 #else
 	(void)user;

src/daemon.h

@@ -23,7 +23,7 @@
 #include <stdbool.h>
 
 void
-daemonize_init(const char *user, const char *pidfile);
+daemonize_init(const char *user, const char *group, const char *pidfile);
 
 void
 daemonize_finish(void);

src/main.c

@@ -94,6 +94,7 @@ static void
 glue_daemonize_init(const struct options *options)
 {
 	daemonize_init(config_get_string(CONF_USER, NULL),
+		       config_get_string(CONF_GROUP, NULL),
 		       config_get_path(CONF_PID_FILE));
 
 	if (options->kill)